The invention relates to a method and apparatus for coupling the color television signal recorded on a magnetic tape to a studio synchronizer for the purpose of reproducing the color television picture represented by the signal. More particularly, the invention relates to a color television signal recorded on magnetic tape of the type wherein the chrominance signal is frequency-translated downwards into a frequency range lower than the frequency range of the luminance signal. Still more particularly, the invention relates to the problem of eliminating time base errors in the color television signal derived from the magnetic tape.
For the recording of TV signals onto magnetic tape for the purpose of subsequent reproduction, certain recording methods have come into common use. In particular, the high relative speeds between the magnetic head and the magnetic tape requisite for the recording of the high-frequency signal components are in general achieved by making use of magnetic heads mounted on the periphery of a magnetic head wheel turning at high speed and writing onto successive tracks oriented perpendicular to or inclined relative to the direction of travel of the magnetic tape.
A transverse-track recording method of this type making use of an approximately 50 mm magnetic tape is characterized by high resolution, a high signal-to-noise ratio, and good image stability. However, the equipment employed is complicated and expensive and, furthermore, by reason of the relatively great breadth of the magnetic tape, is very space-comsuming, as a result of which such system has hitherto found use only in TV studios.
To a growing degree, resort is had to a recording method making use of inclined tracks, according to which a significantly smaller magnetic tape is coiled around a longitudinally subdivided cylinder having a region of subdivision in which turns the magnetic head wheel provided with the magnetic heads. The magnetic tape, after the recording operation, comprises successive track sections which extend from one to the other longitudinal edge of the magnetic tape, at a very small angle to the direction of elongation of the tape, with each track section in general containing the signals representing one half image. In general, the tape, in addition to the portion thereof taken up by the tracks for the video signals, is furthermore comprised of a longitudinally extending audio signal track, as well as a control signal track onto which there are recorded, during the recording process, synchronizing signals for correctly synchronizing the operation of the magnetic head wheel and the magnetic tape drive during subsequent reproduction of the recorded TV picture.
As a result of the relatively small dimensions of the inclined-track magnetic tape apparatuses, new areas of application are opening up to the magnetic recording of TV signals. For example, portable battery-energized devices have been developed which in conjunction with a portable TV camera have made possible on-the-spot TV recording of events without regard to location. However, the subsequent reproduction of the recorded-on-the-spot TV signals by means of telecast equipment places demands upon the mechanical and electrical characteristics of the magnetic tape apparatuses almost as stringent as those which are customary for studio equipment. Even when, as a result of the smaller relative speed of movement between the magnetic heads and the magnetic tape, a lower resolution is to be tolerated, synchronization errors, which disadvantageously result in fluctuations of the horizontal image holding action, still must be suppressed or avoided.
The reproduction of color TV signals places even greater demands upon time stability, because with color TV signals even much smaller time errors will result in phase fluctuations in the TV signal leading to unacceptable color errors in the reproduced color TV picture.
The frequency of the conventional color subcarrier in the European PAL color television system is approximately 4.433 MHz. However, due to the limitation of bandwidth resulting from reduction in the speed of relative movement between the video heads and the magnetic tape in the magnetic tape apparatuses referred to above, signals in this frequency range cannot be satisfactorily recorded. It is known to frequency-translate downwards the chrominance signal into a frequency range below the lower sideband of the luminance signal. Known devices translate the chrominance subcarrier down to a frequency of 562.5 KHz with sidebands extending from 0 to about 1 MHz. Accordingly, the chrominance signal will occupy a frequency range lower than the frequency range of the FM luminance signal and, in this frequency range, can with very little interference be recorded onto magnetic tape using recording equipment of the type referred to above.
Additionally, in the event of phase fluctuations, the phase shift to which such signals are susceptible is reduced by an amount equal to the ratio of the frequency of the downwardly-translated chrominance subcarrier (i.e., 562.5 KHz) to the conventional chrominance subcarrier frequency in the European PAL system (i.e., 4.433 MHz).
Considered in absolute terms, the time base errors of TV signals derived from the magnetic head wheel of an inclined-track magnetic tape machine of simple design are about one order of magnitude higher than those of signals derived from the magnetic head wheel of a studio-type magnetic tape machine.
Accordingly, immediate coupling of the TV signal derived from an inclined-track magnetic tape machine with a studio synchronizer is not possible. Instead, the time base error of the signal derived from the magnetic tape must be brought down to a value of about 3 nanoseconds or less by means of an electronic phase equalizer, before the studio color carrier can be synchronized with the color carrier derived from the magnetic tape during reproduction of the recorded color TV picture.
A condition indispensable for achieving a standard or studio quality signal is that stringent coupling between the color carrier frequency and the horizontal frequency of the signal derived from the magnetic tape be guaranteed. However, in the known methods referred to above involving downwards frequency-translation of the chrominance signal and pilot carriers, this coupling is lost during the decoding of the TV signal in the reproducing equipment, and it is accordingly not possible to take the composite color television signal derived from such apparatus and subject the signal to a simple phase correction in order to synchronize the signal with the studio synchronizer, as would be done when only studio-quality equipment is involved.